The basic problem of how photic signals are processed by the central nervous system so as to control the timing of reply signals was investigated in the fireflies Photinus pyralis of Maryland and P. concisus of Texas. By means of image-intensified videoscopy in the field and recording photometry in the laboratory it was shown in P. pyralis that flashes perceived between 4.5 and 6s after the male's flash (normal free run period 6s) evoke an immediate response (latency 400 ms). As a result, several neighbor males may become self-entrained into essentially synchronized flashing. P. concisus shows a comparable male-male triggering in both field and laboratory (latency 650 ms) but, unexpectedly, shows also an alternative response with the same latency as the free run flashing period (2.2 s). P. concisus, therefore, can be entrained by a rhythmic series of artificial signals in much the same way as certain Oriental firefly species (and human beings), although over a narrower frequency range. Flash control timing thus shows unexpected complexity, raising questions about the previously accepted dichotomy between directly triggered and resettable pacemakers.